1. Field of the Invention
The present invention relates to a semiconductor device having incorporated therein a circuit for outputting a power ON reset signal.
2. Description of the Related Art
At the time of turning ON the power of a semiconductor device, in order to avoid malfunction, it is necessary to sense that a value of a power supply voltage enters an operating range, and then, make an initializing operation based on a sense signal. A sense voltage for sensing a power supply voltage always needs to be set so as to be equal to or smaller than an operating guarantee voltage and must be at a value for all circuits to normally operate. Sensing a power supply voltage includes: a method utilizing a threshold voltage of a transistor or a method utilizing a charging and a discharging of a capicitance. However, with either of these methods, it is unavoidable that the sense voltage is distorted depending on process distortion or temperature characteristics.
In the case where a rise of the power supply voltage is extremely slow, the initializing operation is made under the sense voltage. Alternatively, in the case where the distortion of the sense voltage is large, there is a danger that the voltage is lower than a lower limit in an operating range of a circuit requiring the initializing operation, and it is desired that a precise voltage should be sensed.
In particular, in the case of a semiconductor device having multiple power sources, an operation is started after sensing that all the power sources have been turned ON. However, all the power sources are not always turned ON at the same time, and thus, there occurs a case in which the initializing operation is performed while one power source is at a maximum operating voltage and another power source is equal to or smaller than an operating guarantee voltage. In the case where a circuit targeted for initialization uses a plurality of power sources, the circuit is required to operate in a wider voltage range by a distortion of a power sensing circuit than a normal operating voltage, making a circuit design difficult in particular.
In order to precisely sense a power supply voltage, it is preferable that the power supply voltage should be compared with a reference voltage generated from a Band Gap Reference (BGR) circuit or the like. However, when a value of the power supply voltage is low, a level of the reference voltage itself cannot be guaranteed, thus making it necessary to take countermeasures such as increasing a power supply voltage of a reference voltage generating circuit. Therefore, there is a problem that a circuit becomes complicated and large-scaled.
In Jpn. Pat. Appln. KOKAI Publication No. 11-344533, there is disclosed a semiconductor test device comprising sequence control means for, in power units of the semiconductor test device for sequentially making sequence control of rises of a plurality of unit power sources in order of first, second, and third unit power sources, when the first unit power source should supply a direct current voltage to a load unit earlier than the second unit power source, detecting that the direct current voltage outputted by the first unit power source has reached a predetermined value or the like, and outputting a direct current voltage of the second unit power source.